Intense efforts in recent years have surrounded work in improving transmitter efficiency, especially for mobile terminals (User Equipment, UE) to extend battery life. It is also an area of focus for network equipment to reduce heat dissipation. Given that most of the transmitter efficiency as well as RF performances are dominated by the power amplifier (PA), various techniques have been implemented. This has included Polar PA architecture, Doherty PA design, PA output load tuning, and others.
One of the most notable and has been steadily gaining attention as the method of choice is Envelope Tracking (ET). This technique describes a varying bias voltage for the PA driven by a custom designed modulator, which tracks the envelope of the RF signal to be transmitted at the output of the PA. The purpose is to reduce bias overhead (i.e., power overhead) to what is necessary to amplify the Radio Frequency (RF) signal. This results in significant Power Added Efficiency (PAE) improvement on the PA but at the expense of linearity, which is traditionally achieved from bias overhead that we have removed with ET.
As such, some form of pre-distortion techniques need to accompany the implementation of ET, the most common being Digital Pre-Distortion (DPD). Linearization is necessary for ET transmitter to meet Adjacent Channel Leakage Ratio (ACLR)/Adjacent Channel Power Ratio (ACPR) mask requirements. However, in order for DPD to work the envelope signal and the transmission signal path signal must be time aligned at the ET transmitter. Otherwise, instantaneous waveform clipping by envelope misalignment will cause non-linearity.